The National Transportation Safety Board released probable cause reports on Friday for two fatal accidents that occurred during the hunting season in the fall of 2013. In both cases, pilot error was found to be the cause, though the circumstances surrounding each crash differed.
Pilot Scott Mueller and passenger Traeger Anderson were departing Tatitna Airport, about 50 miles southeast of Nikolai, in a Super Cub owned by Mueller when they crashed on Aug. 30. According to the NTSB, the aircraft was both excessively heavy and improperly loaded.
Ultimately, Mueller's decision to "load the airplane beyond its allowable takeoff weight and center of gravity limits ... resulted in a loss of control during the initial climb," the NTSB wrote. Other contributing factors were the external load and downwind takeoff. A downwind takeoff -- a takeoff with the wind coming from behind the aircraft -- requires additional runway distance to obtain the necessary airspeed to maintain flight. In this case, the downwind takeoff, along with the load issues, proved insurmountable.
Less than two weeks later, on Sept. 9, Kenneth Whedbee was piloting his home-built Zenith CH-701 airplane with passenger Jason Scott when they crashed shortly after takeoff from Kucera Residence Airport, which is about 4 miles northwest of Big Lake.
The NTSB determined that the probable cause in that accident was "the pilot's failure to maintain adequate airspeed while maneuvering at a low altitude, which resulted in a stall and subsequent spin from which he could not recover."
Visual meteorological conditions prevailed at the time of both crashes and weather was not deemed a factor in either accident.
A heavily loaded aircraft
In the first crash, Mueller and Anderson departed Tatitna about 8:20 p.m. destined for Big Lake. Based on information provided by Anderson and from the crash site, investigators determined the aircraft was approximately 642 pounds over gross weight. The estimated center of gravity at the time of the accident was positive 25.07 inches. The CG range at maximum gross weight for the Super Cub is positive 14.0 inches to positive 20.0 inches. CG limits are set by the design limits of the aircraft and are a primary part of every pilot's load planning. A CG that far aft of the proper range resulted in the aircraft being tail-heavy and thus unable to maintain proper attitude when it became airborne.
In interviews with the NTSB, family members stated that Mueller was returning home after a successful caribou hunt. He had flown multiple trips already that day, shuttling passengers, meat and gear from Tatitna to Big Lake.
According to Anderson, they had just refueled the main wing tanks and loaded two butchered caribou and some gear when they departed. After clearing the treetops, they initiated a left turn and Anderson then heard the pilot say, "I should have taken off the other way."
Anderson, who received serious injuries, reported no recollection of the accident itself.
Through an on-scene investigation, aircraft damage was found indicative of an aircraft departure stall. The NTSB determined the aircraft impacted in a "near vertical attitude." Both propeller blades were attached; one exhibited "extensive leading edge gouges, substantial torsional 'S' twisting and chordwise scratching." The other exhibited aft bending "approximately eight inches from tip." Flight control continuity was verified from primary flight control surfaces to the cockpit.
Scattered throughout the wreckage were meat and hunting gear, including a set of caribou antlers between the left wing lift struts and a rifle still attached to the right wing lift struts. Anderson informed the investigators that there were carcasses from two caribou on board, estimated at 500 pounds total. Based on the weight of the fuel, the pilot and passenger, oil, the rifle and antlers, the NTSB conservatively determined the gross weight of the aircraft at the time of takeoff was 2,392.25 pounds, far exceeding the maximum gross weight of 1,750 pounds. The additional weight of the hunting gear destroyed in the post-impact fire was unknown.
The report made special note of the external load of the caribou antlers and the rifle. The aircraft was not approved to carry external loads, and federal aviation regulations do not permit external loads with passengers on board regardless of possible approval. While not uncommon in Alaska, particularly on float planes, external loads will affect the airflow and thus aircraft control.
A low-altitude loss of control
In the investigation of Whedbee's accident, a family member informed the NTSB that a male grizzly bear had left tracks on the family's private runway for several years. After hearing a report that the grizzly was seen protecting a moose kill in the area, the family member stated that Whedbee decided to fly out and see if he could locate the bear.
Investigators found that the Zenith impacted the ground in a near-vertical attitude on a mass of fallen trees and branches. It came to rest upright, in a nose-low attitude with the throttle found near the full-forward position. A survey of the propeller found that "one of the three composite propeller blades remained attached to the hub assembly, was straight and relatively free of impact damage. The second propeller blade separated near the hub assembly, was bent forward about midspan, and exhibited leading edge gouging and chordwise scratching. The third propeller blade separated near the hub assembly, and exhibited light chordwise scratching."
Damage like this indicates the aircraft was under power and stopped suddenly with the crash, with one blade absorbing most of the impact and separating as it struck the ground. This is common in aircraft stall and spin accidents.
A test run was performed on the Zenith's engine on Nov. 7. The engine remained attached to the fuselage and when fuel was introduced, it started "without hesitation or stumbling, and ran to full throttle." The investigators found no anomalies that would have prevented "normal operation and production of rated horsepower."
"Loss of control at low altitude" was a main topic of discussion at last winter's Alaskan Air Safety Foundation seminar, "Doing the Right Thing." One of the main ways participants discussed avoiding control issues was to conduct careful weight and balance calculations prior to takeoff.
Reach Colleen Mondor at firstname.lastname@example.org
By COLLEEN MONDOR